When I inquiried, a few plumbers and my regular A/C technician told me that they see a lot of problems with tankless water heaters - maintenance-wise. They do not recommend them for that reason and because their cost vs. a regular water heater or heat pump doesn't make sense. But again, every situation is different, so yours may prove that a tankless is the way to go.
My motivation for starting this thread is that people often choose tankless because they want a more efficient water heater. The improvement over a regular water heater is a <10% reduction in energy use vs >70% reduction with a heat pump.
In addition to the higher maintenance costs there's a loss of demand response potential. The ability to control demand is going to get more critical as wind and solar generation expand. Few people would be willing to time their showers around wind generation but with a large tanked heater you sync its heat cycle to when surplus wind or solar is available without any inconvenience. The worst thing for the grid would be a consistent spike every evening and morning when people with tankless electric heaters take showers....